The aluminum nitride has an excellent electrically insulating property and a high thermal conductivity, and it has been expected that the materials such as resin, grease, adhesive and coating material filled with a sintered product thereof or a powder thereof can be used as heat radiating materials featuring a high thermal conductivity.
In order to improve the thermal conductivity of the heat radiating materials, it is important to densely fill the matrix such as resin with a filler having a high thermal conductivity. For this purpose, it has been strongly urged to provide an aluminum nitride powder of a spherical shape having a particle diameter of from about several microns to about several tens of microns.
The aluminum nitride powder has, usually, been produced by an alumina reductive nitridation method which reductively nitrides a composition of alumina and carbon, a direct nitridation method which reacts aluminum directly with nitrogen, and a gas-phase method which reacts alkylaluminum with ammonia and, thereafter, heats them. Of them, the aluminum nitride powders obtained by the reductive nitridation method and by the gas-phase method have shapes close to a sphere but their particle diameters are still of the order of sub-microns.
According to the direct nitridation method, on the other hand, the aluminum nitride powder is obtained through the pulverization and classification, making it relatively easy to control the particle diameter and to obtain the aluminum nitride powder having particle diameters of from about several microns to about several tens of microns. However, the particles constituting the powder are angular non-spherical particles. Therefore, the aluminum nitride powder obtained by the above method cannot be highly densely filled in the resin.
Therefore, a variety of methods have been studied in order to obtain the aluminum nitride powder of a spherical shape having a desired particle diameter.
For example, a patent document 1 discloses a method of obtaining an aluminum nitride powder of a round shape having an average particle diameter of not less than 3 μm by firing a mixture of an alumina powder and a carbon powder in an inert atmosphere to form an aluminum carbide permitting particles thereof to grow and then firing the particles thereof in a non-oxidizing atmosphere containing nitrogen. This method, however, involves the conversion of the firing atmosphere making it difficult to control the growth of alumina particles, i.e., making it difficult to control the particle size distribution of the obtained aluminum nitride powder.
Further, a patent document 2 discloses a method of producing a spherical aluminum nitride powder having an average particle diameter of not more than 50 μm, a sphericalness of not less than 0.8 and excellent water-resisting property by reductively nitriding a spherical alumina with a nitrogen gas or an ammonia gas in the presence of carbon and, thereafter, oxidizing the surfaces thereof. According to this production method, however, the spherical shape of the starting alumina becomes directly the shape of the aluminum nitride powder which is the final product, and it is necessary to use the alumina having a large particle diameter equal to the desired particle diameter. With the alumina having such a large particle diameter being reductively nitrided, however, the reaction must be conducted for an extended period of time to improve the conversion. As a result, the oxygen concentration increases in the obtained aluminum nitride, and thermal conductivity decreases accordingly.
On the other hand, a patent document 3 discloses a method of producing an aluminum nitride powder by using, as a starting material, a mixed powder of an aluminum oxide powder, a carbon powder and an alkaline earth metal compound or a rare earth element compound, and firing the mixed powder in a non-oxidizing atmosphere containing nitrogen. This method is to form the aluminum nitride at a low temperature of not higher than 1,500° C. by utilizing the action of the alkaline earth metal compound or the rare earth compound for accelerating the reaction. However, the aluminum nitride powder obtained by this method has a particle diameter of, concretely speaking, about 1 μm, and relatively large particle diameters of the order of several microns have not been realized yet.
Further, a patent document 4 discloses a method of obtaining a crystalline aluminum nitride powder by ripening (heat-treating) an amorphous aluminum nitride powder in a flux comprising compounds of alkaline earth elements or rare earth elements so as to assume a spherical shape and, thereafter, dissolving the flux to isolate the crystalline aluminum nitride powder. This production method is capable of achieving a high fluidity and a high filling ratio permitting, however, impurities to easily infiltrate into the aluminum nitride powder requiring, therefore, strict control of production conditions, i.e., increasing number of the steps and driving up the cost of production.